CYCLIN-A EXPRESSION IN NORMAL AND TRANSFORMED ALVEOLAR EPITHELIAL-CELLS

The mature adult alveolar epithelial cell (AEC) is a highly differenti ated phenotype that does not readily divide and exhibits numerous spec ialized functions. Yet, transformed AEC proliferate agressively in cer tain forms of lung cancer. Normal AEC also proliferate but in a coordi nated manner during embryonic growth and fetal development as well as during lung repair. Therefore, biochemical mechanisms regulating the c ell cycle in AEC are clearly of fundamental significance for understan ding lung development, lung injury, and cancer. Cyclin A is a protein that varies in abundance during the cell cycle and regulates critical transition points through its association with cyclin-dependent protei n kinase subunits. We postulated that high expression of cyclin A migh t be associated with rapid proliferation in transformed AEC. We compar ed the expression of cyclin A mRNA and protein in primary cultures of fetal and adult rat AEC, in the E1A-T2 neonatal rat AEC, and in the ma lignant A549 human AEC. We used pharmacologic blockades with mimosine, aphidicolin, and nocodazole for cell cycle synchronization, which was verified by fluorescence-activated cell sorter (FACS) analysis of cel lular DNA content. Transformed cells (A549 and E1A-T2) exhibited a muc h higher level of expression for both cyclin A mRNA and protein than d id normal rat AEC. Induction of cyclin A mRNA expression in A549 human AEC and E1A-T2 rat AEC occurred in late G1, prior to the onset of S p hase. Fetal and adult rat AEC and rat E1A-T2 AEC expressed two cyclin A mRNA transcripts, whereas human A549 cells in S phase and M phase ex pressed three cyclin A mRNA transcripts. We conclude that transformed AEC overexpress cyclin A in comparison with primary AEC cultures, whil e retaining cell cycle-dependent differences in cyclin A expression. W e speculate that cyclin A expression is regulated both at the transcri ptional and post-transcriptional levels, and that cyclin A may play a key role in the increased proliferation of transformed AEC that is ass ociated with the pathogenesis of lung cancer.